This invention relates generally to hot runner injection molding and particularly to injection molding apparatus having an improved mold core.
Cooling the mold cores of hot runner injection molding systems to reduce cycle time is well known. Reducing cycle time by even a fraction of a second is very important in large volume applications such as making beverage bottle preforms. As seen in U.S. Pat. No. 5,094,603 to the applicant which issued Mar. 10, 1992, this has normally been done by circulating cooling water through and around a central cooling tube in the mold core. While this arrangement is satisfactory for many applications, it generally does not provide sufficient cooling. Another problem with the central cooling tube is that if it is not precisely centered or there is any other reason for uneven cooling of the melt in the cavity, the uneven cooling will produce uneven forces in the cavity which can shift the core. This results in the preform not having uniform wall thickness which causes problems in the blowing of the preform.
U.S. Pat. No. 5,498,150 to Check which issued Mar. 12, 1996 does show a mold core having an outer part with longitudinally extending grooves therein. While this does increase the cooling surface area, there is still a considerable delay waiting for the melt to solidify before the mold can be opened for ejection. Also, as the mold core forms a part of the cavity, it must have sufficient structural strength to withstand injection pressures of up to 10,000 psi and this limits the size of the cooling water conduits. Furthermore, the flow of cooling water through the mold core is generally laminar which reduces the efficiency of cooling of the melt. Mold-Masters brochure entitled "Introducing Master-Stack Closure Molding Components" shows a mold core having cooling fluid bores extending radially outward in the head. This is suitable for molding closures but not for molding preforms.